1. Field of the Invention
This invention in general relates to photographic optical systems and in particular to a zone focusing optical system particularly suitable for use in a camera of the type which includes an automatic ranging and focusing arrangement.
2. Description of the Prior Art
According to the well-known Gaussion lens equation, an image of an object which is located ahead of an optical system will only be sharply formed on film located behind the optical system when the distance between the optical system and the focal length of the system is correct. Since the location of film in photographic cameras is typically fixed, this means that sharpest imaging at a given object distance requires a unique focal length or a unique optical system to film distance. The process of adjusting the optical system focal length, or the optical system to film distance, to achieve maximum image sharpness at different object distances is known as focusing and may be accomplished in well-known ways. One way of focusing, for example, is to provide a lens of fixed focal length in combination with a means for adjusting the spacing between the lens and film as a function of object distance. The usual arrangements for accomplishing this involve the use of a flexible bellows connecting the lens mount to the film retaining housing and a linkage for extending the bellows to alter the spacing between the optical system and film.
Another known focusing arrangement utilizes a variable focus optical system fixed in place ahead of the film. Here the lens focal length is changed with object distance by changing the air spacing between individual elements of the lens, usually by moving a front cell of the lens with respect to other elements or an element.
Another known focusing arrangement suitable for use with fixed lens to film distance situations involves the use of a rotatable lens turret which carries a plurality of lenses having different focal lengths each of which can be rotated into place along the camera taking path. These arrangements can be classified as zone focusing systems because they do not provide a continuously variable focal length but rather provide discrete focal lengths which sharply image for particular object distances and more or less sharply image objects located on either side of that object distance most appropriate for the particular lens focal length. In this manner, such systems provide adequate sharpness over a range or "zone" of object distances relying on the assumption that for any given focal length there exists a blur, due to defocus, of small enough size such that the performance of the system will not be adversely affected provided that the object remains in the appropriate zone. An example of such a system is shown and described in, for instance, U.S. Pat. No. 3,418,908 issued to Edwin H. Land on Dec. 31, 1968 and entitled "Range Finding-Focusing Apparatus For A Photographic Camera".
It is also known to provide zone focusing arrangements by combining a fixed lens with a turret having lenses which can be selectively indexed into alignment with the fixed lens such that combinations of the fixed lens with individual turret lenses provide a series of different focal lengths. One such arrangement which utilizes a fixed doublet is described in U.S. Pat. No. 494,128 issued to Erskine Decker on Mar. 28, 1893 and entitled "Lens For Cameras".
Of the three focusing arrangements described, the first two have the advantage of continuous adjustability over a range of object distances. However, for a particular application this advantage must be evaluated in view of the increased cost associated with the need for bellows extension arrangements or more complex multi-element lens systems. Zone focusing systems, while not offering the continuous adjustability feature, are often adequate and offer an attractive alternate because they can generally be made less expensively, than the others. However, most known zone focusing systems are somewhat cumbersome, requiring a large amount of space because of their turret arrangements. Therefore, it is a primary object of the present invention to provide a zone focusing system which is inexpensive, compact and easy to manufacture.
It is another object of this invention to provide a zone focusing optical system that is particularly suitable for use with a camera of the type which includes a sonar ranging system and an arrangement for automatically positioning a lens disk as a function of select object distances.
Other objects of the invention in part will be obvious and in part will appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed description.